red flag

something that provokes an angry or hostile reaction: The talk about raising taxes was a red flag to many voters.

Also called powder flag. Nautical. a red burgee, designating in the International Code of Signals the letter “B,” flown by itself to show that a vessel is carrying, loading, or discharging explosives or highly inflammable material.

(initial capital letters)a war game the U.S. Air Force holds several times each year at Nellis Air Force Base, Nevada, to train personnel in air combat.

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Origin of red flag

First recorded in 1770–80

red-flag

[red-flag]

verb (used with object),red-flagged,red-flag·ging.

to mark or draw attention to for a particular purpose: The department has red-flagged the most urgent repair work to be done.

to provoke the attention of; alert; arouse: The animal's refusal to eat red-flagged the keeper that something was wrong.